The invention relates to a frequency-selective signal receiver equipped with a voice-frequency trap and employed in communication equipment, more particularly telephone equipment, having a clipping circuit at the input thereof, wherein the signals to be received consist of several simultaneously appearing single frequencies.
The difficulty in receiving purely audio-frequency code dialing signals consists in screening out undesired signals by means of a voice-frequency trap that attenuates undesired signals but does not suppress the desired signals; such desired signals may be accompanied by noise levels caused by dial tones, announcements or other noise. In prior art devices of this type the noise level for each group of frequencies is fed to a common noise detector in the voice-frequency trap sections of the signal receivers. As a result, the interpreting threshold is allocated to all frequency channels so that individual frequency trapping for each frequency channel is not possible. Particularly threatened channels i.e. channels particularly likely to carry undesired signals cannot be monitored according to their exposure to danger. Moreover, the reciprocal action of the frequency groups caused by the finite stopband attenuation of the group filters cannot be taken into account.
Frequency-selective signal receivers equipped with a voice-frequency trap have been proposed in the past wherein rectangular voltages derived from the signal frequencies are fed sequentially in cycles to a common detector circuit, and wherein the particular frequency is determined through counting; thus the counting circuit for determining the frequency is also part of a voice-frequency trap circuit which compares the count values for two consecutive half waves. The disadvantage of such signal receivers resides in the fact that detector circuits must be utilized having a receiver channel with a very narrow bandwidth for each single frequency occurring so as to ensure adequate voice frequency trapping. Due to the use of such narrow-band receiver channels, comparatively long transient response times are required which render rapid character recognition impossible.